Refactoring: Introduce ResouceLoaderHost interface for unloading ResourceLoader.

This moves several direct dependencies of ResourceLoader into
newly inroduced ResouceLoaderHost interface, which is implemented by
CachedResourceLoader.

The motivation here is to make ResourceLoader working without DocumentLoader
and FrameLoader so that scripts can be loaded through imported documents, which
doesn't have a DocumentLoader.

Note that CachedResourceLoader already lives without DocumentLoader in some case,
thus it is natural to localize such kind of complication there and keep ResourceLoader
straightforward.

BUG=240592
TEST=none
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/19574002

git-svn-id: svn://svn.chromium.org/blink/trunk@154452 bbb929c8-8fbe-4397-9dbb-9b2b20218538